Ultra-high-frequency tuning apparatus



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FREQUENCY TUNING APPARATUS ULTRA-HIGH- 2 Sheets-Sheet 1 Filed Dec.

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Fwd/0 ma) m'mw in 65% 646ml, 7 @W Patented Jan. 23, 1945ULTRA-HIGH-FREQUENCY TUNING APPARATUS Eduard Karplus, Belmont, andArnold P. G.

Peterson, Cambridge, Mass, assignors to General Radio Company,Cambridge, Mass., a corporation of Massachusetts Application December10, 1941, Serial No. 422,348

9 Claims.

The present invention relates to ultra-highfrequency tuning apparatus.

The principal object of the present invention is to provideultra-high-frequency tuning apparatus which is readily tunable over awide frequency range, and which avoids the difficulties of erraticoperation encountered in some existing resonant circuit controls.

With this and other objects in view, the present invention comprises theapparatus hereinafter described and particularly defined in the claims.

In the accompanying drawings, Fig. l is an elevation of one form of unitaccording to the present invention; Fig. 2 is a sectional detail view online 2--2 of Fig. 1; Fig. 3 is a detail view similar to Fig. 2 butshowing elements in different proportions; Figs. 4 and 5 are elevationsof a modified form of apparatus; Fig. 6 is a sectional detail view online 6-6 of Fig. 4; Fig. 7 is an elevation of a modification; Figs. 8and 9 are views illustrating the use of a tuning unit with an oscillatortube; Fig. 10 is a sectional view showing two tuning units used with atube; and Fig. 11 is a diagram of the circuit for the apparatus of Fig.10.

The construction shown in Fig. 1 comprises a complete tuning unitsuitable for use with an ultra-high-frequency oscillator. It includessimultaneously variable capacitive and inductive elements. The unitcomprises two stator members l6, each generally in the shape of aquadrant. The two stators are mechanically supported by an annular bandi8, to which the stator plates are soldered. The band may be consideredas constituting the inductive element of the unit. A rotor element I9 isin capacitive relation to the stators.

The frequency range for which the unit is used is determined by the sizeof the band l8 and the number and size of the condenser plates [6. Foran extremely high frequency range, the arrangement shown in Fig. 2 maybe used, wherein each stator comprises two plates 20 and the band I8 isrelatively deep, whereby the unit has small capacitance and inductance.The rotor element i9 is constructed either as a solid or hollow assemblyhaving only slight clearance with the stator plates.

For a lower frequency range, the construction shown in Fig. 3 may beused, wherein the stator is made up of a larger number of plates 22 togive higher capacitance, and the connecting band, shown at I8, isconsiderably narrower to increase the inductance. The rotor, here shownat 19', has a series of stacked plates adapted to interleave with theplates of the stators I6.

In either form, the rotor is mounted on a shaft 24, which is preferablyof insulating material. The rotor is concentric with the band l8 andarranged so that its outer edge has only slight clearance with respectto the inside of the band. The rotor is adjustable from the position ofFig. 1 through an angle of to a position in which the rotor isinterleaved to the maximum extent with the stator elements.

The unit may be viewed as involving two condensers in series (eachcondenser including one of the stators and the rotor) and the inductanceformed by the band l8, said inductance being parallel with the seriescondensers. Actually the electric and magnetic field distributions arevery complex, particularly for intermediate positions of the rotor, butthe foregoing will serve for an explanation of the operation of thedevice. In the closed position, wherein the rotor is completelyinterleaved with the stator plates, both the capacitance and theinductance are at a maximum. In the position of Fig. 1, which may betermed open position, the inductance and the capacitance are at aminimum. The reduction in inductance is effected by the restriction ofarea for the passage of magnetic flux within the band l8. It will beunderstood that at ultra-high-frequencies, the magnetic flux will passthrough only the space not occupied by the metal rotor. For open andpartially open positions, therefore, the flux will be constrained topass through the restricted clearance space between the band l8 and therotor, and the inductance will be markedly less than in the closedposition of the rotor.

The simultaneous variation of both capacitance and inductance affords awide tuning range. Moreover, this variation is attained without the useof sliding contacts or other connections to moving parts. Suchconnections have been a source of erratic operation in high-frequencyresonant circuits. For example, when the rotor shaft is used as acurrent conducting element, the passage of stray currents through themetallic bearing surfaces may cause erratic operation. In the presentinvention, the rotor shaft may be of insulating material, so that therotor may be said to float, and the bearings may be placed well outsidethe field, since their only function is to provide mechanical supportfor the rotor.

Figs. 4, 5 and 6 show a modified form of the invention in which opposedstator members 28 of quadrantal form are connected by inductive bandelements 30. A rotor assembly 32, having opposed plates of generallysector shape, is mounted on a shaft 34, concentric with the bands 30.The

stator may be constructed as shown in Figs. 2 and 3, by soldering ofplates to the band, but a modified arrangement is shown in Fig. 6,whereby the stationary member is built up of died-out plates of propershape with intermediate annular spacers 36, the several plates andspacers being secured by screws 38.

The construction of the above-described modification is symmetrical.Electrically, it is essentially the same as theiconstruction of Fig. 1,except that two parallel inductive paths are connected between thestator elements. As in Fig. 1, both the inductance and capacitance arevaried from minimum (Fig. 4) to maximum (Fig. 5) by movement of therotor. The entire range in this case is covered by a rotor adjustment of90, The symmetrical construction is usually to be preferred over theconstruction of Fig. 1, because it can be constructed with greatermechanical rigidity; a150, it results in smaller stray fields.

In Fig. 7 is shown a modification of the construction last described,wherein the rotor plates are shaped as indicated at 48 instead of havinga circular periphery. The purpose of this shaping is to produce adesired relationship between tuning frequency and angle of variation.For example, in the unit of Fig. 4, the curve of the logarithm offrequency plotted against angle of rotor movement is approximately astraight line. This particular characteristic is desirable instandard-frequency oscillators. Substantially any desired characteristicmay be obtained by proper shaping of the plates. It will be understood,how ever, that any departure of the rotor plates from true circular formresults in a diminished inductive and capacitive variation, and hence ina diminished tuning range.

The devices heretofore described may be used wherever a variableresonant circuit is required, for example, in an absorption typewavemeter. They also find frequent use in conjunction with thermionictubes, as oscillators, etc. In such cases, the tube elements (grid andplate) are preferably connected to the maximum potential points of thecircuits, namely, to the stator elements. Such connections should be asshort and direct as possible, to reduce the lead inductances' betweenthe tube elements and the high-potential points of the resonantcircuits. The actual mounting arrangement for the tube will depend onthe tube design. Two different tube mountings are shown in Figs. 8 to10.

An arrangement involving the use of the tuning element with a so-calledacorn type tube is shown in Figs. 8 and 9. Mounting brackets 42 areattached to the opposed stators 28, and terminate in ledges 44 tosupport the grid and plate prongs 46 and 48 of the tube. The tube prongsare held in position by spring clips 58. Since the brackets 42 are bothat the same potential, so far as direct current, is concerned, it isnecessary to insulate the grid and plate prongs from each other. This isconveniently accomplished by means of a piece of mica or otherinsulating material 52 placed under one of the brackets 42. At the highfrequencies for which the unit is tuned, the insulation 52 introduces nomore than a negligible impedance.

The cathode terminals 54 are mounted on suitable blocks 56 on thecontrol panel 58, through which the shaft 34 passes. The tuning assemblyis mounted on the panel by means of supporting posts 68 preferably ofinsulating material.

In Fig. 10 is shown another arrangement employing an oscillator tube ofdifferent type. The tube is shown at 62, and is provided with plateterminals 64 protruding from both sides and grid terminals 65 alsoprotruding from both sides. This type of tube, when used in anultra-high frequency system, has usually been placed at the center of ahalf-wave transmission line made up of concentric or spaced parallelconductors. The transmission line forms an efiicient circuit, but doesnot lend itself to adjustment. According to the present invention, thetube is connected in a circuit employing one or two of the tuning unitspreviously described. As shown in Fig. 10, the tube is placed betweentwo tuning units 68, each of the type shown in Fig. 4, mounted in ametal enclosure 10 by means of supporting columns 12. The rotor elementsare adjusted simultaneously by means of a manually operated shaft 14connected through gears '16 at both ends with the shafts of the separateunits. It has been found necessary to shield the tuning units 68 fromeach other, and this is accomplished by means of a metal shield 82having an opening of .proper size to receive the tube. Insulatingspacers 84 are provided in the supporting columns 12 and the shafts 80.

The plate terminals of the tube are directly connected to the tuningunits by mounting blocks 85, and the grid terminals are supported onmounting blocks 86 insulated from the tuning units by pieces of mica inthe same manner as indicated at 52 in .Fig. 8. For simplicity, thecathode connections are omitted from Fig. 10.

The electrical circuit for the system of Fig. 10 is shown in Fig. 11.The condenser stators are diagrammatically indicated at 28 and therotors at 32. The inductance elements 38 are indicated in theconventional way by coils 30. The diagram is intended to indicate thatvariation of the rotor setting changes both the inductance andcapacitance. The plate of the tube is connected through its opposedterminals to the stators 28 of the two units, and the grid is shown asconnected to the symmetrically opposed stators through fixed condensersindicated at 52. The grid is connected through a choke and resistor 88to ground The plate supply is connected through chokes 88 with themid-points of the inductance elements 30. These mid-points are nodal orzero-potential points when the rotors are in fully open or fully closedposition. At other rotor positions, the nodes may shift slightly, sothat the chokes may 7 be necessary to confine the high-frequencycurrents to the resonant circuit. The cathode of the tube is connectedto ground. The internal capacitances of the tube are indicated in dottedlines. The system oscillates at a frequency dependent on the rotorsetting. The feed-back coupling necessary for oscillation is furnishedby the inter-electrode capacitances of the tube.

The system shown in Figs. 10 and 11 is bilateral, in that it affordssymmetrical connec tions for the tube terminals. A uni-lateral systemmay be used by omitting one of the tuning units, but the bi-lateralarrangement, as shown in the drawings, is preferable because theinterelectrode capacitances are divided between the two resonantcircuits, whereby a higher tuned frequency may be reached.

The diagram of Fig. 11, although directly applicable to the apparatus ofFig. 10, will serve to indicate an electrical circuit suitable for usein conjunction with any of the units herein described.

The invention is useful for ultra-high-frequency applications, forexample, in circuits resonant to frequencies extending roughly from 30to over 1000 megacycles per second.

It is to be understood that the invention is not limited to the preciseembodiments herein described, but may be varied within the scope of theappended claims. For example, in some instances it has been found thatsupporting posts of metal may be used, particularly when the posts arelong and the inductive loop is small. In such a case the posts provide aparallel inductive path, but at very high frequencies the inductance maybe o high as to have a negligible effect.

The resonant circuit may be connected with a tube to form an oscillatorin ways other than those shown. Instead of grounding the cathode, eitherplate or grid of the tube can be placed at ground potential for radiofrequencies by grounding the corresponding part of the tuned circuit.Although the arrangement herein described with no electrical connectionsto the rotor is to be preferred, connections to the rotor may be made,if necessary or desirable, as for example, when the rotor is to begrounded or connected to a tube element. It will be understood that therelatively large circulating current in the resonant circuit would notbe required to pass through the rotor connections, and the advantagesheretofore noted would be obtained.

Having thus described our invention, we claim:

1. An ultra-high-frequency tuning unit comprising a pair of condenserstators of generally quadrantal shape, a complete annular bandsurrounding and supporting the stators and form ingparallel inductivepaths between them, and a rotor having quadrantal elements in capacitiverelation to the stator and in inductive relation to the band and movableto vary both the capacitance and inductance of the unit.

2. An ultra-high-frequency tuning unit comprising a pair of condenserstators of generally sectorial shape, an annular band connecting andsupporting the stators and forming parallel inductive paths, a rotorhaving elements in capacitive relation to the stator and. in inductiverelation to the band and movable to vary both the capacitance andinductance of the unit, and a shaft of insulating material for therotor.

3. An ultra-high-frequency tuned circuit comprising a plurality ofcondenser stators, terminals on the stators, an inductive elementconnected to the separate stators and forming between them the inductivepath of the tuned circuit, a movable element mounted for movement withrespect tothe stators and inductive element but not conductivelyconnected therewith, the inductive element being disposed substantiallyin a plane and the movable element being movable in said plane, themovable element in closed position having a substantial surface opposedto the stator surfaces, and in open position substantially unopposed tothe stator surfaces and in increased proximity to but out of contactwith the inductive element to form with said inductive element a gap ofreduced area for passage of magnetic flux, whereby the circuit may bevaried from maximum capacitance and inductance between the terminals inclosed position of the movable element to minimum capacitance andinductance between the terminals in open position of the movableelement.

4. An ultra-high-frequency tuned circuit comprising a plurality ofcondenser stators, terminals on the stators, a looped inductive elementconnected to the separate stators and forming between them the inductivepath of the tuned circuit, a rotor mounted ior movement with respect tothe stator and inductive element, the inductive element being disposedessentially in a plane and the rotor being movable about an axisperpendicular to said Plane, the rotor in closed position having asubstantial surface opposed to the stator surfaces, and in open positionsubstantially unopposed to the stator surfaces and enclosed within butout of contact with the inductive element to form with said inductiveelement a gap of reduced area for passage of magnetic flux, whereby thecircuit may be varied from maximum capacitance and inductance betweenthe terminals in closed position to minimum capacitance and inductancebetween the terminals in open position of the rotor.

5. An ultra-high-frequency tuned circuit comprising a plurality ofcondenser stators, a band disposed essentially in a plane and connectedto the separate stators and enclosing a free area, said band formingbetween the stators the inductive path of the tuned circuit, a movableelement mounted for movement in said plane with respect to the statorsand the inductive element between an open position and a closedposition, the movable element having a substantial surface opposed tothe stators in closed position and being substantially entirely includedwithin said free area in open position, whereby th circuit may be variedfrom maximum capacitance and inductance in closed position to minimumcapacitance and inductance in open position.

6. An ultra-high-frequenc-y tuned circuit comprising a. pluralityofcondenser stators, an annular band disposed essentially in a plane andconnected to the separate stators and enclosing a free area, said bandforming between the stators the inductive path of the tuned circuit, arotor mounted for movement with respect to the stators and the inductiveelement about an axis perpendicular to said plane between an openposition and a closed position, the rotor having a substantial surfaceopposed to the stators in closed position and being substantiallyentirely included within said free area in open position, whereby thecircuit may be varied from maximum capacitance and inductance in closedposition to minimum capacitance and inductance in open position.

'7. An ultra-high-frequency tuned circuit cornprising a pair ofdiametrically opposed stators of sectorial shape, an inductive elementcomprising an annular band disposed essentially in a plane and connectedto the separate stators and enclosing a free area and forming betweenthe stators the inductive path of the tuned circuit, a rotor havingplates mounted to inter-leave with the stators for maximum inductanceand capacitance and to lie within said free area for minimum inductanceand capacitance, and means for mounting the rotor for adjustment aboutan axis perpendicular to said plane to either of said positions andother positions intermediate thereto.

8. An ultra-high-frequency tuned circuit comprising a plurality ofcondenser stators of generally sectorial shape, an inductive elementcomprising an annular band disposed'essentially in a plane and connectedto the separate stators and enclosing a free area and forming betweenthe stators the inductive path of the tuned circuit, terminals on thestators, and a rotor having a portion movable about an axisperpendicular to said plane, between a closed position in which it has alarge capacitive relation to the stators and an open position in whichit occupies the free area with reduced capacitance and inductancebetween terminals.

9. A variable circuit for providing a resonant ultra-high-frequencyimpedance comprising two terminals, a symmetrical pair oi bands forminginductive paths between the terminals and providing a closed contour, arotor movable within the contour between a position of maximum resonantfrequency and a position of minimum resonant frequency, the bands beingdisposed essen- 1o

